| /* |
| * Copyright 2012-present Facebook, Inc. |
| * |
| * Licensed under the Apache License, Version 2.0 (the "License"); you may |
| * not use this file except in compliance with the License. You may obtain |
| * a copy of the License at |
| * |
| * http://www.apache.org/licenses/LICENSE-2.0 |
| * |
| * Unless required by applicable law or agreed to in writing, software |
| * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT |
| * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the |
| * License for the specific language governing permissions and limitations |
| * under the License. |
| */ |
| |
| package com.facebook.buck.graph; |
| |
| import com.google.common.base.Preconditions; |
| import com.google.common.base.Predicate; |
| import com.google.common.collect.ImmutableList; |
| import com.google.common.collect.Iterables; |
| import com.google.common.collect.Maps; |
| import com.google.common.collect.Queues; |
| import com.google.common.collect.Sets; |
| |
| import java.util.Map; |
| import java.util.Queue; |
| import java.util.Set; |
| import java.util.concurrent.atomic.AtomicInteger; |
| |
| public class TopologicalSort { |
| |
| private TopologicalSort() {} |
| |
| public static <T extends Comparable<?>> ImmutableList<T> sort( |
| TraversableGraph<T> graph, |
| final Predicate<T> inclusionPredicate) { |
| |
| // AtomicInteger is used to decrement the integer value in-place. |
| Map<T, AtomicInteger> effectiveOutDegreesOfExplorableNodes = Maps.newHashMap(); |
| Queue<T> nextLevel = Queues.newArrayDeque(graph.getNodesWithNoOutgoingEdges()); |
| Set<T> visitedNodes = Sets.newHashSet(); |
| ImmutableList.Builder<T> toReturn = ImmutableList.builder(); |
| |
| while (!nextLevel.isEmpty()) { |
| Queue<T> toExplore = nextLevel; |
| nextLevel = Queues.newArrayDeque(); |
| |
| Set<T> level = Sets.newTreeSet(); |
| |
| while (!toExplore.isEmpty()) { |
| T node = toExplore.remove(); |
| Preconditions.checkState( |
| !visitedNodes.contains(node), |
| "The queue of nodes to explore should not contain a node that has already been" + |
| " visited."); |
| |
| if (inclusionPredicate.apply(node)) { |
| level.add(node); |
| } |
| visitedNodes.add(node); |
| |
| // Only add a node to the set of nodes to be explored if all the nodes it depends on have |
| // been visited already. We achieve the same by keeping track of the out degrees of |
| // explorable nodes. After visiting a node, decrement the out degree of each of its parent |
| // node. When the out degree reaches zero, it is safe to add that node to the list of nodes |
| // to explore next. |
| for (T exploreCandidate : graph.getIncomingNodesFor(node)) { |
| if (!effectiveOutDegreesOfExplorableNodes.containsKey(exploreCandidate)) { |
| effectiveOutDegreesOfExplorableNodes.put( |
| exploreCandidate, |
| new AtomicInteger(Iterables.size(graph.getOutgoingNodesFor(exploreCandidate)))); |
| } |
| if (effectiveOutDegreesOfExplorableNodes.get(exploreCandidate).decrementAndGet() == 0) { |
| nextLevel.add(exploreCandidate); |
| } |
| } |
| } |
| toReturn.addAll(level); |
| } |
| |
| return toReturn.build(); |
| } |
| } |